I am looking for a good English-Thai dictionary to do with Biology.......can anyone point me in the right direction, please ?

It's for a Thai friend who lives overseas, who is studying nutrient science.

You could try the Kinokuniya bookshop in Bangkok.
Alternatively ask at a university such as KKU in Khon Kaen.
I'm sure there must be an online site.
Probably not much help but you might strike lucky.

I'm not aware of any Thai-English biology dictionaries, but there are quite a few medical ones which might do the job.  Here's one that's available for on-line purchase:

 

https://books.google.co.th/books?id=n1lSDwAAQBAJ

 

This presentation has images of the covers of six such dictionaries on page 4:

 

http://www.med.nu.ac.th/pathology/humanities/File2557/3.Medical Dictionary.pdf