I'm gonna take a guess .
What amp/speaker configuration does your friend have? I did a quick search for marshall MG50 dfx and it seems to be a 1x12" combo. Do you have your amp on the floor? The feedback is caused by the sound from the speaker(s) looping back through your guitar pickups so if you have your MG50 on the floor, the speaker is that far away from your pickups. If your friend had, for example, a 4x12" half or full stack... much closer to the pickups.
...again, just a guess, could be something entirely unrelated
Oh, I forgot to ask... same guitar on both amps? Could be the guitar/pickups?