screen replacement oem or aftermarket?

Hi my son has a 6s and recently had a screen replacement done at a reputable phone repairer and since it has been done the phone is playing up it keeps freezing and this makes it unusable and requires a forced shut down or it just shuts itself down occasionally. Also there has been 2 white glow kind of lights showing coming from the bottom of the screen that were not there before, I have taken the phone back to them 4 times now in the past week and their response is that the phone doesn't' like the aftermarket screen it is all the phone's fault and for an extra $$$ they will put a OEM one on and that will fix it all. I feel we are being taken for a ride any ideas or advice would be great. the phone belongs to my son and he has saved for weeks to get his broken screen fixed also the phone never missed a beat at all before they touched it he had only had it for about 4 months and it was broken for that whole time and never had a problem. advice please has anyone heard of phones not liking aftermarket screens