Retired runner interference?

[shipwreck]

I had a NFHS varsity game the other day. Runner on first and defense tries to turn a double play. The runner going from first to second is plunked by the ball when she is only halfway to second base. I did not rule interference on the retired runner but maybe I should have. The rule says if a retired runner interferes the runner closest to home would be declared out. I didn't know what the declared runner could do since she was only halfway between first and second but maybe that does not come into consideration.