Go with her to the clinic.
(My $0.02, but it's probably not worth that much as it's just advice from a theoretical perspective....)
Don't be overt about telling your wife about it yet. She needs someone she can trust and you're it. Let her decide when/how she tells her mom.
Go with her to the clinic so that it gets treated and you know how she's supposed to care for it.
Amy makes some good suggestions but be discrete until your wife is on the same page.
Once things settle down, impress upon her the risks of her behavior. Somehow. :-(
Best of luck!
Cheers,
Scott.
