The guitarist may be highly experienced with many gigs under his belt with the conference to improvise. If this is the case though I would hope he would be able to signal his changes to the rest of the band and have the maturity to guide the less experienced members so they were working with him and nailing the songs in rehearsal.
I haven’t played in a band for a long time and when I was in one we were all equally inept so stuck rigidly to the structure. We were fairly rubbish but I saw plenty of local bands live who were technically more proficient than us but sucked live as they were doing exactly what your guitarist does.
It was like watching 10 year olds in a local football tournament. 11 would be strikers all doing their own thing.