Bitcoin mining is considered environmentally unfriendly because it uses fossil fuel energy. If crypto or bitcoin mining uses solar energy or water, surely the mining will be environmentally friendly. In my opinion, in addition to the proof of stake system, using a proof of work system like bitcoin is also not a problem as long as it uses environmentally friendly renewable energy.
There is no requirement in Bitcoin mining industry that electricity should be generated specifically from the fossil fuels. The miners use electricity from the national grid and therefore it is not their responsibility to check whether the power generation is done using renewable sources or not. If the environmental activists are so concerned, then they should request the government to shut down all the coal-fired power plants and pay double price for energy that is generated from solar or wind powerplants.