Under MMP the number of seats a party has in parliament is determined by the party vote, not the electorate results (unless they win more electorate seats than they would get under the party vote - this is called an overhang). Winning Epsom wouldn't likely give National an extra seat.
National's party vote in this election gave them 60 seats, whatever happened in Epsom. If National had won Epsom they would still only get 60 seats (one less National List MP would get in, replaced by the Epsom electorate MP). This means that their welfare towards ACT effectively gives National
an extra seat that their party vote wouldn't otherwise give them. If ACT didn't win Epsom their 0.6% of the vote would be spread across all parties, so it might mean a different party got an extra seat instead.
If your question regards to the 0.6% of the party vote that ACT got and whether National would be better off killing them and taking their party vote the following election, that a) assumes that vote would go to National which isn't necessarily assured, b) assumes no other far right wing party would spring up in their place and c) requires a level of long-term thinking that would be fairly unusual among our politicians.