Is a friend who combines religiosity with leniency in some matters, such as mockery and justifying error, considered a good or bad friend?

Good companionship is required both by religious law and by human nature. A good companion is a great gain, and no one can be described as such unless he adheres to religion and possesses noble character. Your description of your colleague as an imam of a mosque, a حافظ (Hafiz, memorizer) of the Quran, and someone concerned with his religious affairs, these are good qualities. Perfection is rare, and who among us is free from mistakes? Your companionship should be based on sincere advice and reminding, for "religion is advice." If you are both like that, then be keen on his companionship.