Can Kelp and alfalfa speed metabolism. I am very skinny so this not about me. But a male friend of mine takes it.

However, he is feeling a little lightheaded when he goes into certain light so is this a side effect of kelp and/or alfalfa

... not a side effect of kelp/alfalfa.

It's a side effect of eating NOTHING but kelp/alfalfa.

... is this friend not eating?

If he has a thyroid problem then kelp can help or hinder it depending on if his thyroid has too much or too little iodine/
What his thyroid problem is

He could try this test.
How to Self-Test for an Iodine Deficiency

If he has enough iodine he should stop the kelp until he checks with the doctor about it.

I have heard that alfalfa sprouts are not good I don't know about them in supplement form