Having read the responses, it seems that some people have advised for him to see a doctor and not really said why. While I'm not a doctor, from my studies and experience, I can say the following.
If his ejaculate is a yellow/tan/burnt color there is definately a foriegn matter in there. From my studies, the deeper the color, the more severe of a problem/higher concentration of foriegn particulate there is in the fluid.
I'll list some of the ill causes for yellow semen and then some of the normal causes.
Some examples of things that will turn the ejaculate yellow. Smoking Pot yes, Bacterial infection of either the Prostate and or Testicles, Viral infection of the Prostate and or Testicles. If this is the case, these infections are VERY serious. In means that there was either a STD or more simple UTI that spread into the prostate and beyond. He would likely want to have this checked (I forget how old he was and if he'd been with anyone else. However, it's possible to get an infection of the prostate in non sexual ways. No judgement here, just observation)
There are a number of very normal and harmless reasons for yellowing of the ejaculate as well. (I'm being specific on Ejacualte, because there is Semenal Fluid, Semen, and a multitude of other secretions in the ejaculate)
The first reason could be that he has a highly productive Prostate. There is an enzyme produced in the Prostate that is yellow in color as well. In some men, this yellow can ball up or form rice shaped clumps. It is NOT the normal but within standards of normal production. Also, not sure how long he would go between orgasms, but length of time can also deepen the color of the ejaculate.
As far as fear, I see no reason to fear, but I would tell my little sister not to swallow this semen unless he got checked out. Better always to be safe.
PS. Just thought of something else, if there is a prostate problem (ie infection or swelling of the prostate, that can prevent proper contraction and would result in weak ejaculation. It can also be as another gentleman said, lack of PC muscle strength) I'd ask him to get a check up and be as specific as he can be to the doctors.