Is concealing testimony considered a sin, in the event that there are witnesses to an assault, knowing that the testimony might lead to a greater tribulation, and what is the way to be rid of this testimony, and is lying in it considered a sin?

If the young man's claim depends on this testimony, then the witness must provide it, unless he fears harm upon himself; due to Allah's saying, "Let no scribe be harmed or any witness," and the Prophet's (peace be upon him) saying, "There should be neither harming nor reciprocating harm," and because he is not obligated to harm himself for the benefit of another. If the testimony leads to discord, then it is not obligatory for him if he fears harm.