I'm with the group that thinks this is probably not a priority issue if the band has no plans to do anything but jam together.
If you really do want the band to improve/evolve, my recommendation would be to see if there is some type of bigger goal you can convince everyone to work toward. Maybe you want to float the idea of playing one or two gigs far out in the future, or do a small recording session and/or record a more high quality demo for limited circulation. Basically anything that gives some goal that doesn't push everyone too far but gives some excuse for things to tighten up. Part of the appeal of this is you can try to pitch the idea of everyone coming together a little more seriously without having to single out the lead guitarist as much. However, this obviously only works if everyone is on board; if people just want to jam and have fun the way things are, that's probably what they will do, including the lead guitarist.
If you really do want the band to improve/evolve, my recommendation would be to see if there is some type of bigger goal you can convince everyone to work toward. Maybe you want to float the idea of playing one or two gigs far out in the future, or do a small recording session and/or record a more high quality demo for limited circulation. Basically anything that gives some goal that doesn't push everyone too far but gives some excuse for things to tighten up. Part of the appeal of this is you can try to pitch the idea of everyone coming together a little more seriously without having to single out the lead guitarist as much. However, this obviously only works if everyone is on board; if people just want to jam and have fun the way things are, that's probably what they will do, including the lead guitarist.