Welcome FrankReade: It's good that your friend is in therapy and has a desire to get better and work on things.
My question is should I tell him I think he has BPD? I think he would want to know, but don't think it is really my place to tell him and think he may react badly. It's just hard because he is asking for my help and would say he wants to know, but his therapists (who on their profiles list BPD as an experience area) have not told him.
Several of the traits for BPD are found in normal people in varying degrees (and in various situations). It's only when there is a grouping of a high number of them, that a personality disorder is diagnosed. The best approach is to not get hung up on the diagnosis. What matters is that the therapist tries to help with the management of the traits/symptoms.
You don't want to be the person to try and diagnose him. Sometimes, therapists avoid labels, as they can have a negative effect. The best way to approach things with him is to deal with specific behaviors and leading him towards coping skills. Mindfulness is a coping skill and can be a tool for anyone, but it is especially helpful for someone with BPD of an anxiety disorder.
The therapist is likely trying to coach him to learn some skills to self-soothe. Various forms of meditation, mindfulness and breathing techniques are some of the coping tools that can help people who have BPD, anxiety disorders and other mental disorders/illnesses.
One type of therapy that promotes mindfulness is ACT Therapy. My therapist practices ACT Therapy and recommended the book, The Happiness Trap by Russ Harris. I found that I could access some online material to assist with participating in some exercises suggested in The Happiness Trap book.
EXERCISES - FROM BOOK" THE HAPPINESS TRAP"https://www.thehappinesstrap.com/free_resourcesYou don't want to take on the task to fix him, only he can do that. Perhaps, you might suggest to your friend that he inquire about the therapy style his therapist uses. Perhaps he could ask his therapist to recommend a helpful book for him. Since he seems to be seeking your support, perhaps you could support him and help coach him in learning some coping skills. (with the overall guidance of his therapist). So, if the therapist recommends a book, with some exercises to work on, you could help you friend with his homework (reinforce or support it in some way)
What is typical is that people with BPD or BPD traits, is that they tend to unload their emotions with people they feel most comfortable with. When there is a romantic relationship, the partner tends to be the one who is the recipient of the worst behavior. Hence, his bad breakup
Are there some behavioral issues that your friend exhibits with you?
It would be beneficial for you to set some
BOUNDARIES with him. Boundaries are for your protection and are up to you to consistently enforce them.
The skill of validation/not invalidating can be a helpful tool for you to use with your friend. Don't invalidate his feelings (by word, body language or expression). The lessons at the links below can be helpful.
VALIDATION - DON'T INVALIDATEINVALIDATION EXAMPLES VALIDATION - LEVELS OF VALIDATIONIf you are able to share some details, we can suggest some more communication skills.