My friend fasts for two consecutive months, so is this permissible or not?

The best voluntary fasts are those prescribed by the Messenger of Allah, peace and blessings be upon him, such as fasting on Mondays and Thursdays, Ashura, Arafat, three days from every month, six days from Shawwal, the entire month of Muharram, and most of Sha'ban. The best fast for those who regularly observe it is fasting one day and breaking fast the next, which is the fast of Dawud, peace be upon him, because it is the most equitable fast and the gentlest on the soul and body. It is permissible to fast for a month or more consecutively, as long as it does not lead to physical weakness or inability to perform obligatory duties. If it affects one's ability to perform obligations, then breaking the fast becomes obligatory, because a supererogatory act should not lead to the neglect of an obligatory one.