General

Depends, and not the underwear lol.

Does she know about your preference, and is she ok with her weight? If yes, go for it. Seems pretty safe. It gets murkier if she has body issues.

I know I love lingerie!

"bursting the pants and ripped panties" is so hot.

It depends if she wants to gain or not.

I love it, when I try on a pair of my wife's briefs and they are tight on me, or when I can not even get them on!
Usually these full cut briefs have plenty of room, since they are for a bbw.
She will tease, and make fun of me, when I can not pull them up. Calling out how fat I have gotten, by pointing out my upper thigh fat deposit on my inside thigh.

When I am in bed with a female friend, I tend to check not necessarily discreetly her bra size and most of them accept it.

I have read somewhere that it is nice to buy a smaller size pants/bottoms to show you thing she is smaller and a larger size bra to show you think her breasts are longer.