Welcome NeedSupportPls: Sorry to hear about the difficulty with your son's partner. Whether his partner can wear the label of BPD or not doesn't really matter. What matters most, is to learn communication skills to manage the way you interact with his partner and react to her. You are wise to come here and prepare for the future.
My question is, how can I present this information to my son without alienating him? I do not want to butt in on their relationship, yet feel it is my duty to make sure my son has the information he needs in order to make educated decisions. His partner's high functioning makes it a bit dicier. . .When I ask my son about it, he makes excuses, says she's not like this when I'm not around, and shrugs his shoulders. The co-dependency was obvious
That's a tough situation. People with BPD (pwBPD) commonly engage in a behavior called
SPLITTING . She sees you as a threat and perhaps a need to compete with you for your son. Right now, your son is probably idealized and "painted white", while you might be on the opposite side of the spectrum and "painted black".
You say your son is codependent. That seems to be a common situation. Many times the son will take the partner's side, especially while he is "painted white". Those who are codependent are apt to join forces with their partner and eliminate their FOO from their life, and appease the pwBPD in all regards. Be careful. If you son says he doesn't see issues with his partner, he won't likely relate to what you might try to share about BPD. Some codependents might share what you say with their partner, or their partner might catch wind of your thoughts in some way (overhearing a phone conversation, monitoring your son's email, texts, etc. Some codependent don't keep much private, if they have a jealous partner.
Be careful of what you do and say. You are better off to NOT disagree with any position his partner takes.  :)on't
JADE (Justify, Argue, Defend or Explain).
Be very careful of social media. If you give your son and his partner (or anyone associated with them) access to your Facebook or other shared media, do not disagree with anything they say or will read and perhaps interpret as some slight or disagreement with anything connected with them. Keep with the logic that if you don't have something nice to say, don't say anything.
They are talking about buying a house, a further enmeshment. What's a mother to do?
I appreciate you concern. Buying a house with friend or partner, outside of marriage, has lots of potential for disaster. I've been through that experience and learned some lessons the hard way. No matter how much you want to rescue your son from a disaster, it isn't likely possible.
Others might have different opinions to share. Unfortunately, you will likely be walking on eggshells around your son's partner. Since he is codependent, you have to be careful with him as well.
Do you have any thoughts about some possible root cause of your son's co-dependence? Is it an issue he admits to? Has he tried to address the problem?
On an individual basis, many of her behaviors can be excused, yet the overall pattern is unmistakeable.
What are the behaviors that can be excused, versus what is unmistakable
Do BPD's tend to stay stable at whatever level they function, or can they deteriorate?
One size doesn't fit all, but stressful event or life changes can bring on some undesirable behavior. I've heard several stories that center around a wedding and how a MIL is painted very black after the wedding. After the wedding, there can be lots of opportunity for problems. You can't expect to be able to easily work through the types of disagreements you might have with more normal people. So, the best plan will to be to prevent disputes, disagreements, opinions, etc. You won't usually have anything you way or get apologies. One logic to use is, "decide what hill do you want to die on" - meaning before you JADE something, be sure that it is so important to you, that you are prepared for the consequences of a rage, the silent treatment, painted black or blacker, etc.