I think knowing and being able to trust the guy is the first thing, then I'd want testing for everyone before any contact, then no outside fucking around.
Open yourself up before fucking with dildos until you can take one a little bigger that his cock.
Oh yes it can, it'll stretch a mile before it tears an inch. 😄
You might try hinting around about it just to get some insights about how she feels about it. When I told my hot gf I was sucking my gay friends big cock she asked if she could join in. Surprised the hell out of me.
I've had my side lovers thick 8" cut cock all the way in when we'd fuck missionary. Very hot,I loved it,took it all the way without any pain. I felt like a total whore.