A few biosecurity-oriented answers and opportunities:

• The Next Generation Global Health Security Network has a mentorship program with plenty of mentors and support from outside the developed world.
• The BWC has run a Biosecurity Diplomacy Workshop for Young Scientists from the Global South for the past two years.
• The G7 Global Partnership is also currently working in partnership with Africa CDC on a signature initiative to enhance biosecurity in Africa.
• IFBA has a global mentorship program. Although these certifications aren't focused on GCBR mitigation, they can end up giving you a role in pandemic response. Here's one example where someone was the one (1) person in her country who was able to certify biosafety cabinets, which were important for COVID testing.

This old post of mine on "Effective altruism in non-high-income countries" and the links in the comments might be helpful.

I made a comment on the Reddit post just now - hope that helps!

Here is an old, informal 80,000 Hours document on this topic: https://wiki.80000hours.org/index.php/Potentially_promising_career_paths_in_poorer_countries

If he’s seeking to work directly in Gambia on development, perhaps becoming an agricultural researcher or adviser - like an ag extension agent - would be a way for an individual to help many people in his community. According to Access Gambia’s agriculture page, 80% of the population work in agriculture. Considering the difficulty of the short rainy season, maybe he could identify additional crops, specific cultivars, cover crops, co-plantings, or techniques well-suited to the local particulars that would increase agricultural productivity.

http://www.accessgambia.com/information/farming-agriculture.html