Good for you, man! That's a hell of a development, and sounds like honestly, a fun challenge for both of you. https://youtu.be/lgadDML8QJ4?si=JKCgFgHRWdSwOAY_
So, I'd suggest introducing protein shakes in her diet. If she's going to be working out semi regularly, recommend you guys grab some packs/powder of protein shakes... You could go a couple ways with this. With my partner, we just integrated them into her schedule, and she has one of those single-serving cartons of a protein shake with her morning tea/coffee, followed by her breakfast, and eating everything else normally. That really helped keep her base caloric intake every day to be almost always at a surplus. A win for her, is that she feels better in the mornings, and it helps her build muscle much easier, without having obsess with intake and diet. Simple, works, can be expensive to buy so many packs of those cartons, though.
Second option, if you guys are playing into sabotage, you could buy the powder and make shakes for her. That way, you guys could engage in making her shakes even more calorically dense (with her consent, of course). Could always prepare her morning beverages for her, so she'll never know if you're making a standard shake, or one that would make her gain a pound just looking at it. There are definitely ways to do it without changing the flavor/viscosity too much, too i.e. changing out milk for Ripple protein milks, real peanut butter instead of PB powder, chocolate syrup instead of cocoa powder, etc. you can play and have fun with it.
Another thing you can start doing is more of a passive effect, but just absolutely LOAD your home up with her favorite treats every time you go shopping. This one's extra effective if she's a weed smoker, in which case you can also maximize that by looking for strains for her high in myrcene and CBN.
Take her out to restaurants and encourage her to go nuts. Sometimes, it's all a matter of suggestion. If you guys aren't usually the type to go for apps or desserts, suggest things you'd think she'd like especially. Again, the more you can encourage weed smoking before dates or big dinners, the better.
Also, buy her clothes that make her feel nice in her body, and confident in her appearance. If she has a wishlist, grab her some clothes, maybe even size them up one size so she feels comfortable enough to not worry about outgrowing a particular garment, or, if you do this frequently enough, she won't notice her own gain until many dozens of pounds later.
I could go on. Lol