Actually lexapro is one of the prefered anti-depressants for people with Bipolar disorder.
In general, the problem is that in a small percentage of Bipolar people, antidepressants cause them to become manic, thus in most cases making sure the Bipolar patient has enough mood stabilizer is a priority.

Nobody can say how people react to certain drugs. But it seems that the stragey is correct since your friend is already on a mood stabilzer. The antidepressant will help with the depression, sucide impulses and self-harming, but they need to be aware of any signs of mania while giving him the Lexapro. Since he is in the hospital, they can observe that.