TCC with irregular periods


this is going to be a weird post. my sister-in-law (we're thicker than blood, love her) has decided she wants to TTC her first, but she's nervous to do it alone. fortunately, we've decided to TTC our last. issue is, she's very irregular. she goes a few months between periods (her last one was may 9th as an example) she hasn't been tested to find out why because there hasn't been a reason til now. we've gotten her OPKs and a Basil thermometer, is there anything else I can do to help her? anyone with experience with irregularity and had success TTC?