Tell her the truth (as I understand it you don't think she's fat), but also ask why she's asking in the first place. Offer some support if she wants to lose some weight.
I think open and honest discussion is the best approach here because now you're jus guessing what the real issue is.
It's recommended to install YunoHost on fresh, dedicated systems. It'll most likely not break anything but it's best to have backup ready. It is possible to then expose docker containers, but I'd just recommend installing Nextcloud in ynh and transfer the contents over.
Isn't it that NSFW communities and posts therein are implicitly filtered out when browsing in SFW mode?