You want to figure out what boundaries the two of you have about intimacy with the other couple, and what you’re definitely cool with. If any of it is something that’s likely to come up, be sure to talk with them about it so they’re aware - for example, if you decide that deep kissing is only for the two of you, saying so before hand prevents an awkward and confusing stop at a heated moment. Don’t bother looking at ‘polyamory’ resources, you’re not looking to date the other couple so almost all of the stuff that you read about poly won’t be applicable. Reading about ‘swinging’ or ‘open’ relationships will have more relevant information.
As far as the BPD thing goes, if she’s actually getting effective treatment then it’s not a concern - she can use therapy and those techniques to deal with any issues that come up. If she’s not getting effective treatment or goes off of the treatment, nothing that you can possibly do will stop the relationship from a giant crash and burn, so there’s no point in avoiding allegedly ‘risky’ activities. If you can live your life without walking on eggshells, don’t walk on them just for people on the board.
I hear a lot of people say things like this, but it always seems to be based on counting anything bad as a mark against polyamorous relationships, and not counting anything that goes bad against monogamous relationships. I had a poly relationship with an ex-that went badly and had a major blowout at the end, so some people would say ‘see? poly just doesn’t work’… but she also had around ten significant (moved in together, made long-term plans) mono relationships before that that blew up much more spectacularly. At least one ended with a lawsuit, and one ended with a death, but somehow that’s not a data point for ‘mono relationships never end well’. Most people have tons of stories of mono dating and relationshipping gone wrong, but don’t lead from there to 'mono doesn’t work. And the vast majority of relationships that blow up into newsworthy events like murder-suicide or kidnapping kids are mono, but again that falls into the ‘doesn’t count’ category.
There’s also a bias that it’s not uncommon for people who have a breaking mono relationship to decide that some kind of opening of the relationship will fix things, which it doesn’t, but people blame whatever sort of poly or swinging they did for the breakup instead of the underlying issues. Also I don’t count a marriage where people get together, then find they don’t like each other, but stay in the marriage for the kids or social standing or because they’re scared to strike out on their own, so are miserable but unwilling to change as a mono relationship ‘ending well’, but it seems to count for that.
Plus a lot of people in poly/open/swinger/kink/etc relationships keep it in the closet for anyone who’s not running in those circles. I know a number of people who present as a simple, 2.5 kids and picket-fence family to their bio-family, work, and acquaintances, but get up to a bunch of wild stuff in their own times. They typically get counted as a ‘ending well’ mono relationship too. And while researchers find that approximately 25-50% of ‘mono’ relationships have someone cheat over the course of the relationship, people don’t seem to treat that as a failure of mono relationships or as a reason to act like the consequences of having sex with another partner don’t apply to mono relationships.