What bike is this?
Those sliders are normally pretty robust. I've not seen the chain guide come close to melting on any of the XR's or XL's that I've owned. Unless that bike is super high mile, there has got to be something that caused the issue. Could be something as simple as a stuck timing chain link. If it were heat only I would expect other engine damage.
I'm not normally a replace the timing chain whenever you open the engine type of guy, but bobkyle2 might be on to something. The part that failed is the guide that the chain tensioner acts on. The chain may be a bit longer on one part than another. When the long part goes by the guide the tensioner tightens down on the chain. When the short side goes by there is a tremendous force of the guide as most tensioners only move in the tighter direction. No idea if that is the issue, but something caused extra force on the chain to chain guide.
Anyhow be sure to check the chain, the sprockets, and for sure that the tensioner is working and is set correctly.
I've had big heat related problems with cam chain tensioners on my XR600. It's a completely different design though.
good luck and get a manual before doing anything else.