Help - Need guidance about SET communication skills

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Hi -

Not sure if this post is on the right thread.

 Im a newbie here and just getting acquainted with communication skills with my son's UBPDgf.  After no communication for months, I reached out to apologize for being unsympathetic (this is before I discovered BPD family    She responded within an hour to my apology and stated she didn't have a problem with me or my husband, but with our daughter.  GF mentioned several times in the text that she is helpless and doesn't know how it can be fixed (long story). Basically, its all our daughter's fault.

While staying out of the triangle I've been reading about... .

I've studied about using SET communication.  I feel confident about the "S" & "E"  its to "T" i'm worried about messing up.  I'd like to suggest to her to reach out to talk to my daughter. Is that a good idea? I haven't responded because I read its important not to skip the T.   At this point, I'm "walking on eggshells" trying to keep peace for my son's sake.

Does anyone have any helpful suggestions?

Our objective is to better understand the struggles our child faces and to learn the skills to improve our relationship and provide a supportive environment and also improve on our own emotional responses, attitudes and effectiveness as a family leaders

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Can you tell us more about the conflict between the two women?

[ForwardFocused]

I'll try to keep it brief... .it all started when I received a text from gf one night telling me she is moving out of the house she's living in with my son.  She apologized and wanted me to call to check on him.  He said she told him she hated his family and wished they could to move away.  He told her "FU my family comes first"  Naturally, I was shocked and upset not aware there was ever a problem.  My D was present the evening this all went down.  GF moved back in the same night and proceeded to block me and my D from social media.  The next day, she unblocked us.  Since then, my D chose to ignore her, not even acknowledging her at family gatherings. 
I talked with D, suggesting she "just be nice" and stop the childlike behavior.   (D 24yo GF 26yo).  D lives 90 min. away, so they seldom see each other. 
Since my talk with D we went to a baby shower where D attempted to talk to GF and also a wedding where D sat next to GF and attempted to talk.  D thought everything was ok between them until she noticed she had been blocked from all social media the day after the wedding.  GF said D is fake, and was only being nice because her mom was at the baby shower.  D proceeded to confront GF in a text exchange asking her why she blocked her, the conversation escalated until GF told her to F off. 
Note:  At the time all this took place, I was unaware there was even such thing as BPD.  GF has several traits, not sure if there is a dx.  Although I am aware that she was always a challenge as a child and has ADHD.  Knowing what I know now,  we could have responded different.
I also have not shared my suspicions of what is really going on with GF to my D because I don't want to "diagnose" and I'm also fearful of any possible dialog about it getting back to GF or my son.  Not sure how to approach and talk about the situation with my D at this point.  She is really upset to be shut out of their lives.  But, at the same time, not willing to apologize because she said she didn't start the drama to being with... .ugh my family has a huge hole in it... .so heartbreaking... .

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After no communication for months, I reached out to apologize for being unsympathetic (this is before I discovered BPD family    She responded within an hour to my apology and stated she didn't have a problem with me or my husband, but with our daughter... .

This is a comment about BPD, not who is right/wrong in this situation  ... .

Both you and your daughter invalidated her (unitentionally or subconsciously) and she reacted to it.

She took your apology as being sincere. I suspect it is... .you're here 

She took D efforts as insincere. pwBPD can often read others like a book and she is probably reading the your daughters mixed feelings.

pwBPD are impulsive, as you have seen. They can flip back when things seem settled - as she did with you.

So... .

She didn't feel you all embraced her enough in the beginning. Let's face it, most partners feel that (or worry about that) from the in-laws. A person with BPD can be mortally wounded by this.

I suspect that if your daughter makes amends with no pressure on BPDgf to be reciprocal, gives her time, the wound would heal.

I also suspect that if you go above and beyond being inclusive now (but don't flood it of be fake) that she will come around.

What do you think about that?