First, it is important to assure that the woman is in fact experiencing urinary tract infections and to rule out other reasons for vaginal irritation. Please make sure that she has been evaluated by a physician, including the performance of a urine culture (indicated for repeated bouts of infection).
If the diagnosis has been established, there can been a number of causes to cyclic urinary tract infections. As the urethra is located near the vaginal opening, there can been a connection between vaginal irritation and urinary tract infections. In fact, there is a condition described in the medical literature called "honeymoon cystitis" where a woman develops a urinary tract infection due to the frequent intercourse in a less experienced couple. Due to the fact that the UTIs are developing after the couple resumes relations, it may well be due to resuming relations. A first step would be to try using a lubricant to reduce the possibility of irritation.
If using a lubricant does not solve the problem, then she can try reducing bedikot to one per day and seeing if the situation improves. If not, we recommend returning to the physician for further evaluation. At that point, she could consult us or a local halachic authority for further guidance.