Tensioner cross threaded.

Hi, my mate changed the alternator for me on my Disco 300tdi.

A day later, the tensioner on the front of the engine came off and the belt went haywire.

On closer inspection, the bolt had just come out, but its threads are full of whatever the engine block is made out of.

What can I do?

Thanks

10mm recoil or helicoil , fit stud then with loctite ,over time when its undone it often comes out rather han the nut coming off so most havent got the stud thread fully home