No problem! Maybe. She can test a day or two her period is due to find out. Planned parenthood would insert the IUD/nexplanon while she’s on her period and have her do a test, and if she went the pill route they’d have her start at a certain time in her cycle.
Read up on a woman’s menstrual and ovulation cycle in order to educate yourself a bit. There’s a small window in a woman’s cycle where she can get pregnant, but tracking ovulation can be tricky (and thus, the fertile window is larger and there’s more room for error).
If you want to test to relieve anxiety, here’s some cheap ones you can buy in bulk.
AccuMed Pregnancy Test Strips, 25-Count Individually Wrapped Pregnancy Strips, Early Home Detection Pregnancy Test Kit, Clear HCG Test Results, FDA Approved and Over 99% Accurate https://www.amazon.com/dp/B071YNWRPP/ref=cm_sw_r_cp_api_a6z7BbVWKE2AA