An ACL sprain is a type of tear. All ACL injuries can be called sprains or tears. If he has a "tear," that doesn't mean he's out for a year, it depends on the level of the tear. There's three grades, varying in severity. Grade 1 is minor damage and is what people often call a "sprain" (though all the grades involve some tearing)...the knee is still stable and you just need to rest it for a while. Grade 2 is a partial tear and probably means being out for a few months, but surgery may not be required. Grade 3 is a complete tear and the ligament is in two pieces, which requires surgery and is what puts someone out for a year.
So the fact that someone's friend had an "ACL tear" and still played on it probably means he just had a grade 1. If Lillard likewise has a grade 1 tear, it means he's done for this series but there's little to worry about. Grade 2 tears are pretty rare but would also not put next season in jeopardy. It's only if it was a grade 3 tear that he'll miss next season but, if it was a grade 3 tear, he wouldn't have been able to keep playing on it--his knee would have been completely unstable and he couldn't walk properly, let alone run or jump.