Was your brother in law hospitalized with his stroke? If so, was he seen/treated by a Cardiologist? While in the hospital was he sent to Neurosonology to find out exactly what caused the stroke i.e. a blood clot from fibrillation, bleeding or a small bleeding in the brain? Was a CT scan or/and a sonogram done on his brain?
If his "doctor" was unclear as you stated then its high time your brother in law sees a Specialist in that field.
A friend of mine was told by her doctor "she can go to the hospital or stay home" after she had trouble swallowing food and when drinking water it ran out of her mouth and her mouth was crooked. Her doctor said it was a "minor stroke". She believed him and stayed home. Weeks later she had another episode while driving, this time worst, and she drove herself straight to the hospital where they also found out that one of her arteries to her heart was 90% blocked and she had to have bypass surgery.
She is now on massive dose of statins and heart medication.
I've read that atrial fibrillation can cause clots and problems but never heard of tachy of 150 causing much trouble. Perhaps someone else knows. My heart would jump from normal (65) to 180-190 suddenly and stay there for awhile. Then it would just as quickly drop back to normal. It's call PSVT. Now my heart only pops up to 150 or so (after ablations). What's my point? The way I found out about all this funny business was to go to a cardiologist who then referred me to an EP cardiologist (they specialize in rhythm problems). I think your brother-in-law should look into that.
I have had the same problem with no abnormality of my heart structure itself. Never had a stroke and was put on beta blocker to keep my heart rate low.
are they sure the stroke was caused by the tachycardia? I would definitely get a 2nd opinion. Could not hurt