The effects of training and playing at altitude are almost impossible to replicate in the gym. Some people are using oxygen restriction masks to simulate altitude. But since the mask is only worn for a short period, the effects on the blood cell count are non existent. The best advice I would give you is to teach your client about acclimation once they get to the site. You should search “acclimating to elevation for exercise” and/or similar searches to get the information. This is a progression of time and intensity over the duration of the trip. As I don’t know the elevation, length of the trip, current elevation of the client’s location, etc.; it would not be a simple matter for me to lay out all the options.