How can you ask? - Simply mention that you have a loyalty card for the place, it will save 10% off the bill.
Now what is her answer to be, surely it must be "yes"; or she will explain why it is "no".
If she needs to spend a particular amount in order to avoid a clawback such a savings would be unwelcome.
What if her boss takes her there for a annual meet-the-boss lunch or they happen to be in the area, would the boss expect that she would offer her card, and her not connecting this (and not having a card) fails to do so, secretly souring the lunch and meet-and-greet ...
If you are old friends with the recruiter I see no problem brining this up.
If you're not quite that close you don't want to make "yes" a forced answer or "no" a difficult explanation - is it for her benefit (no, she's not paying), it's for your benefit; if her boss is a penny pincher they'll likely appreciate it.
It's somewhat comparable to going on a first date and pulling out coupons.
Nickel and diming is usually best kept out of business discussions as a general rule. Example: You're in an interview and after running the gauntlet they ask what you expect to be paid, you make a fair offer to which they ask if you'd accept a dollar an hour less; sure, for half the workload ...
What can you gain versus what can you lose. If you gain 10 points is it worth 10¢?, if there's anything wrong with the idea it's likely to cost more than 10¢, if you're good friends I foresee no problems.
Asking is easy, when paying the bill comes up simply say you have a loyalty card that would entitle her to 10% off - how this works out is another matter.
