TOPIC:

A friend has been on this for about 3 months.  He experienced diarrhea within the first week and then backed off for a few days.  Resumed one 3 drop dose a day and about a month ago it flared up again.   Quit taking it and 2 weeks later still has diarrhea. 

That doesn't sound right.
Any Herxheimer Reaction should occur and then go away fairly quickly
after you reduce the drops, and especially after you stop completely,
and particularly so after 2 weeks. (usually it's about a day)
AND it should not be occurring if you have already been taking it for 3 months.
(unless you increase the dosage 'significantly')

I don't know if you mean he was actually taking ONE 3-drop dose a day,
or doing 3 drops every hour for 8 hours. (that is Jim's suggested protocol method,
and the one I think is the best)

They should, imo, be following the protocols in Jim Humble's "MMS Health Recovery Guidebook".
It doesn't sound like they were. (?) You weren't real clear. (or maybe you were)
If you Herx, Jim tells you to cut the dosage in half, and keep doing that
until you stop Herxing. Then "slowly" increase (double) the drops until you
get back to the original amount and go from there.

AND you don't just jump in with both feet without looking into deep water.
There is a "Low and Slow" philosophy to follow, starting out slowly with
small drop doses (1/4 drop) and slowly increasing over time (a week or so),
eventually increasing to 3 drops (every hour for 8 hours) if you can do that.
There is no real need to Herx most of the time.

Diarrhea is a fairly strong Herx reaction. Most people get a simple stomach
discomfort, if that. Especially if they do Low and Slow.

So I don't know if you are relaying the message of what your friend did and
has been doing as best as you can and not quite accurately, or what.

Did they take anything for the diarrhea? More than a day or so, depending
on the frequency and severity, is generally not good. You can quickly become
dehydrated and fairly sick.
I usually take Pepto Bismal, although I don't like to, and try not to.

What is their diet?
Are they taking or doing anything else?

It sounds like there might be something else going on.
Everyone is different, of course, but this doesn't sound like a
"simple" Herxheimer Reaction to me.