I feel that your brother-in-law should definitely go to the doctor.
First, It is unlikely that there was an interaction between the medication and the alcohol use. Lexapro is one of many SSRI (selected serotonin reuptake inhibitors) used to treat anxiety and depression. This class of medication is very safe, and though alcohol - especially in the quantity that he drank - is not good for the underlying depression or as an interactive agent with an SSRI, the effects should not be extreme and they should go away within a few days.
On the other hand, this is a lot of alcohol use, even for a person with a current or past alcohol disorder. That he was able to function at all and not have a seizure during the withdrawal is an indication that his condition is quite serious. Second, the symptoms reported (based on this third-hand information) are consistent with either or both of two serious medical problems.
The first and most likely is serious alcohol withdrawal, including blackouts, loss of some short-term memory and any number of strange neurological events such as a new tic or a tremor or a small (or larger) seizure. It is not an exaggeration to say that that level of drinking and stopping suddenly can be truly life threatening or disabling (he could end up with a stroke that would leave him paralyzed).
The second possibility - more remote - is that his "bad patch" and the drinking and the depression are related to an underlying neurological change, possibly a tumor. I do NOT have enough information to give an informed opinion - but it is a possibility. For that reason alone, he should see a doctor.