It looks like your cousin's friend's match could be in the range for a sibling. The total is just under the range as the full siblings in my family. Two full sisters and I have tested, our total ranges from 2400 to 2800 cM (testing different combinations against each other). Testing my husband against his aunt, the total is 1800 cM.

However, the relationship could be half-sibling, perhaps a cousin. Occasionally more or less DNA is inherited since it's random. The numbers quoted above are just averages.

Regardless, it does appear to be a very, very close relationship.