You can pay cash for her, but "treating" has
always been prohibited.
You have to remember that
DDP was originally designed as a lodging promotion, not a dining promotion. That's why it carries those mandatory features of everyone in the room being on DDP for all nights of the stay, etc. Treating someone staying offsite is
precisely the opposite of the intent of DDP.
In the first few years, there was so much abuse of this aspect of DDP (much of it actively encouraged by servers in order to enhance their tips) that Disney Dining had to re-do their accounting software to stop the abuse. Now, the system supposedly won't permit more meals being billed at a particular serving than there are participants in DDP from that room. So if you have 2A/2C on your card, that's all they'll let you bill to DDP -- two adult meals and two childs meals.
And no, you probably can't share. The servers have been threatened with termination if they let people abuse DDP, and it's doubtful they would risk their jobs to allow you to do that. If you're at a buffet, it definitely would not be allowed, because everybody has to pay in some form for buffets.
If it's not feasible to have her stay in your room and pay for DDP for the whole time, I'd take her to a couple of nice meals offsite on the nights she's there. There are a ton of choices offsite for any budget. Then use the TS credits you save those nights to buy yourselves a couple of 2-TS Signature meals.
If you want to have her enjoy a character meal with your family, etc...then just pay the freight. It's not THAT expensive.