my ex girlfriend has two sonos ones and she asked my about the sonos mini sub and if its worth it...i have two 5;s and a mini...

I’m sorry, I don’t understand the setup at all, or frankly, anything in your post. Would you be so kind as to reframe the question for us, and tell us how exactly your ex-girlfriend has her system setup? We can likely, once we can build an image of her system in our minds, help figure out what you’re asking about the Sub mini. 

Subwoofers must be added to a “Room” (Single Speaker, pair of identical speakers, soundbar and surrounds). A subwoofer cannot be operated as a standalone item, it must be “Bonded” to a Room. If a Room is Grouped with other Rooms, any Bonded subwoofers will go along with its Room.

if u buy a subwoofer then its in a room on its own can u then add sonos 1;s too it ...so to make it simple ...ive bought a sonon mini sub then ive turned on two sonos 1;s will on my app it say add sonos 1;s so that its melded and cross wire works (not grouped)

If I understand your post correctly, no. The Sub is always added to a room that already has one (or more) Sonos speakers in it. So, yes, you could add a Sub or Sub mini to a Sonos room that already has PLAY:1s in it, using the ‘add a Sub’ feature in the controller. What you can’t do is add a sub to a room that has no other Sonos speakers, by itself. It must be ‘bonded’ to another Sonos device.