I think you'd be wise to insist that she have a full-panel STD test, and SHOW YOU THE RESULTS, both now and again in 6 months, and you should do the same.
To do any less is playing Russian roulette with your health. Since most waywards cop to one level LESS than their real involvement, there's a high likelihood that the two of them had been physical at some point.
I also think there's no way in Hell she'll DO that, and she'll flip out, but oh well -- it's YOUR HEALTH we're talking about, and I'm not telling you anything that your doctor wouldn't advise similarly. At a MINIMUM, get YOURSELF tested. I did.
