First, I wouldn’t try to diagnos, outside my scope of practice. I’d refer them to a physician.
However, if they’re a runner, I’d be thinking bursitis, a possible stress fracture, or even a nerve compression like Morton’s neuroma. Tendonitis is also a possibility, but bursitis tends to be more common in the foot with runners. It could also be a shoe support issue if they have flat feet or high arches and do not have the correct shoe support or are getting used to new shoes. Pain along the inside of the foot could be a lot of things if going by just that description.
Like I said though, I’d refer them to a doctor to make a diagnosis, even if I suspected to know what the problem may be.