Montgomery plays foil to Prendick, but he's not a straightforward one. He's not the exact opposite of Prendick, nor is he his cell-by-cell clone. These similarities and differences mean Montgomery and Prendick's relationship shifts up and down throughout the story—almost like the waves of the ocean that surrounds them.
For example, Prendick is a teetotaler at first—meaning he won't touch alcohol with a twenty-nine-and-a-half foot pole and hasn't since birth (6.23). In contrast, Montgomery drinks like brandy is liquid life support. It's likely he consumes the way he does because of some deeply imbedded mental anguish—either by helping Moreau in his quest, his mysterious past, or both. Prendick eventually does want a drink after his first run-in with Leopard Man. Montgomery not only provides the brandy but finds "satisfaction" in doing so (10.5). This is an instance of how Montgomery influences Prendick.
Another example is how Montgomery pities and even cares for the Beast Folk. Prendick is likewise affected by Montgomery's regarding the Beast Folk "as almost normal human beings" and begins treating the Beast Folk more favorably. After Montgomery's death, Prendick takes less of a liking to all of the Beast Folk save for Dog Man and the sloth creature. By the end of the novel, Prendick can't even enjoy the company of his fellow human beings (a development more in line with Moreau than Montgomery).