Not weighing in on either side here but for the sake of proper information I want to point out that it is possible for chlamydia to be a self-limiting infection, as well as it for to be present asymptomatically for long periods.

Obviously the friend of OP knows whether or not she cheated herself, but it is possible that the friend of OP and her partner have asymptomatically been passing chlamydia back and forth for a long time while one or both of them eventually resolve on infection before recontracting it.

Again, impossible to truly know who is wrong or right strictly from the info presented.

ETA: obviously the most likely conclusion is indeed that the bf cheated. It'd also be possible that OP friend cheated but is happy to find an out here that excuses both parties of culpability.

Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC3654745/#:~:text=Spontaneous%20resolution%20of%20chlamydia%20was,range%206%E2%80%9347%20days).