I had a look at the map in the area. the 737 must have been flying along Airway UA601, which has the following route: Dakar-Bamako-Tamale-cotonou-malabo. All eastbound.
The HS125 was probably flying ouagadougou UG860 bamako UA601 Dakar, west bound.
FL 350 is an eastbound level, so my GUESS is that the 737 was at least at an appropriate FL for its flight.
Perhaps the HS125 was climbing through FL 350..... Cruising at that level would not be appropriate.