While it is possible there is a path MTU problem, what made you jump to that conclusion?
What troubleshooting have you done? Have you talked to your buddy and explained the problem to them?
Unless you have evidence that its an MTU problem, why are you treating this as if it were an MTU problem?
See The X/Y Problem
If it is an MTU blackhole, usually the symptom is that web sites will open, but then hang when the webpage attempts to send a “full sized” packet.
The mss clamping is a work around to the problem. If you are interested in what it does, watch this video referenced in this blogpost: TCP MSS clamping – what is it and why do we need it? by Ivan Pepelnjak