BasicMed only applies to the PIC, not to a required crewmember (as a safety pilot would be). If you’re acting as a safety pilot and you’re under a BM medical, you are required to be the PIC (and of course, you must meet all the other requirements of actually being the PIC).
See pilot-protection-services.aopa.org/news/2017/september/01/basicmed-and-safety-pilots for more details.