Here's the difference
Wishing & Imitating
For one it’s okay and for other is shirk however the line comes in between
Wishing is Makrooh-E-Tehreemi(MET)
Even if one Imitates without respecting and honouring their celebration, one will not become a Kafir. However this will be MET
What is Makrooh-e-Tahrimi ?
Makrooh-e-Tahrimi is an impermissible act in Deen. It's status is close to status of a haraam act. If one carries out a makrooh-e-tahrimi act, one will be sinful. The consequence of continuously carrying out a makrooh-e-tahrimi act could be disastrous
A non-muslim girl told me yesterday I wish you in Eid but you don't do in our Diwali, Xmas.
Wishing & Imitating is allowed in your religion but it is forbidden in mine
A non muslim comes to masjid by his choice because they don't have any rules but we couldn't go to mandir as
per our restriction
Islam comes with set of rules and boundries. To establish relation is okay but upto some extent.
At last,
"For you is your religion and for me is mine"
Qur'an 109:6
After everything what if a muslim does that sin. S/He wished or s/he participates does she becomes kafir?
See it completely depends on "Niyat"(intention) If one believes there religion is true then the individual is Kafir or the deed is shirk. If not then it is sin goes under MET