Link on the other niche useful?

[Hafsoh]

A link is a link and it will never hurt a site however, the most important thing is that links you get are related, because quality inbound links definitely will help to get higher SERP.

[Thaati]

It won't do any harm to your site in indexing and all. So no issue in getting a backlink from your friend's site. But it won't be of big use when comparing to getting links from sites that are on the same genre of your site

[ELCaD]

all links are useful (providing they aren't from a 'bad area'), but some more so than others. If it is a relevant link then it is going to hold far more weight than a link from a random website, but all links count.

[redirection]

i guess to. someone bought a link in my site for $80/year and his niche is about diamonds and jewelries and my site's niche is celebrity and fashion. and he really insist to put it in my blogroll