If a Muslim believes Christians and Jews to be non-believers then he can never be a true ally to them. Koran forbids such alliances. Read Surah 4 verse 144: “O you have believed; do not take the disbelievers as allies instead of the believers. Do you wish to give Allah against yourselves a clear case?”
In fact a Muslim has a license to pretend as long as he fears the enemy and keeps his heart unaffected. As explains Surah 3 verse 28: “ Let not believers as allies rather than believers. And whoever does that has nothing with Allah, except when taking precaution against them in prudence.”
The only solution is for moderate Muslims to keep reminding young Muslims that Christians and Jews should not be considered non-believers.