You'd need about 2.5to1 pot odds to call.
Example. The pot is $15 he goes all in for $10.
Pot=$25; costs you $10 to call.
He is about 37% to catch his flush.
Out of 100 hands: 37 times you will lose your $10. -$370
Out of 100 hands: 63 times you will win $17.50 from the split pot. $1102.50
Dividing $1102.50 by 100 will give us our average loss or gain after each hand.
$1102.50/100=$11.03. Net gain of $1.03 per hand. +EV

If however, you were only getting 2to1 odds you would lose money.
Example: The pot is $10 and he goes all in for $10.
Pot=$20; costs you $10 to call.
He is about 37% to catch his flush.
Out of 100 hands 37 times you will lose $10. -$370
Out of 100 hands 63 times you will win $15 from the split pot. $945
Dividing $945 by 100 will give us our average loss or gain after each hand.
$945/100=$9.45. Net loss of $0.55 per hand. -EV