Why would the $3k of interest be on your wife’s tax return if the money is in her boyfriend’s account?
Probably more information is missing. If you want to go after the ex I’d recommend passing small costs onto her. For example, have her buy your child a year’s worth of mobile phone service on Mint Mobile. Then have other monthly subscriptions moved to the ex (Netflix, Amazon student, etc). Thirdly, get the ex to research low cost school books for next semester. Lots of opportunities to share costs when a collage of college expenses are taking place.