Mumbai server ping suddenly become ???ms

I have been playing with my friend on Mumbai server for the past 2 months and it has always been 60+ms for me and 100+ms for my friend. Now it has suddenly become ???ms and his ping in game is around 300ms. As for me my ping is good (10-20ms). Does anyone know what happened and how can I fix this? I can't switch to any other servers as the best ping server for my friend was Mumbai.