This PC can move diagonally. Doing so does not involve moving through a hostile creature's square.
(See PHB p.192: Variant: Playing on a Grid)
Characters (and monsters) can move diagonally like they do horizontally and vertically, as a one-square move. From the Entering a Square section we can tell that moving into a diagonal square doesn't involve moving into either of the adjacent squares that border it: with only 5' of movement left you can enter the diagonal square.
Diagonal movements can be restricted by an obstruction that completely fills the adjacent-not-diagonal squares—see the Corners section. But that section specifically calls out terrain and trees—stationary objects, in other words—as the obstruction. A medium-sized creature does not create this type of obstruction: as described on the previous page (Space), the opponent doesn't fill the square it's in.
There is no recognition of the longer path between two diagonal centers than between two truly adjacent centers. The PC can move to any of the corner-spots as one 5' portion of their movement.
The DMG has alternate rules for counting diagonals as alternately 5' and 10' (p. 252), but this only matters for distance considerations—even under that "Optional Rule: Diagonals" your PC could still move to a diagonal space.
Beware opportunity attacks, though!