Is there a risk? Yeah sure. Chlamydia, gonorrhoea , HIV, Syphilis, Herpes and more.
Does she play away from her husband alot and since she tested negative? Keep an eye on any symptoms such as burning and urinating or any usual sores and get tested at 6 week.
Im sure everything will be negative but for peace of mind.
Unprotected sex carries risks of std's. That she probably isn't a high risk person is in your favor but you just never know. Time frames to test and incubation periods for each potential std is going to be different. A health clinic can guide you. HIV, or example if you take a 4th generation duo test can be taken at 28 days and beyond, other tests you can take in a matter of days. https://www.healthline.com/health/how-long-does-it-take-for-std-to-show-up#std-incubation-periods