This should depend on your playthrough I think:
1) Did Ian tell Jeremy that he starts falling for Lena? (If he did, there will be a conflict)
2) Does Ian have good relationship with Jeremy? (If he does, he might believe him that Lena is probably not a relationship material)
3) Did Ian have sex with other girls? (If he did, he might not blame Lena for having a bit of fun as well)
4) Did Ian ask Jeremy for photos of Louise or Alison? (If he did, he might get turned on by the idea)
5) Did Lena sleep with Robert/did Ian fight with Robert at the life drawing event? (see 2, Ian might get disappointed in Lena even more)