Creamer is not really dairy stuff. One way to test is avoid dairy stuff for a week, see if you still get diarrhea or stomach ache. Be sure to avoid spicy and chilli stuff just to be sure.
The other way, the bad test, is to skip breakfast so your stomach is empty, drink a couple glass of fresh milk at lunch and see if you get diarrhea within several hours (usually two hours).
You can also confirm it by clinical tests.

TS sounds like he has frequent diarrhea and stomach ache. I would say maybe he/she has some sort of allergy or intolerance. Of course they could be many reason, and bad diet habit also needs to be factored in.

Considering most Malaysians are actually lactose intolerant with varying degree of tolerance level, TS may have low level tolerance.

The first step though is to identify the cause of diarrhea and stomach ache. A simple step is to reduce your diet to the safest one, no milk, no caffeine, tea or coffee, no gluten, and such. I don't really know the exact steps and all, but I know that's one of the thing you need to look at.