Hi Estaban! I did not have the exact same situation you are describing but wanted to share my experience with you in case it is helpful. While my K1 fiancé visa was pending (I am a Spanish citizen, my now husband is a US citizen), I travelled 4 times to the US under the ESTA to come visit him and never had a problem at the border. I know it is not the exact same scenario you are describing because I used the ESTA and not a student visa but this is to confirm you that your fiancé can travel to the US while the K1 is pending. Last time I entered the US without my K1 visa having been approved, I did it for almost 3 months - which is the maximum amount of time I could stay at a time in the US when traveling with ESTA and the period you mentioned your fiancé program would last - and this time the border officer asked me if I had a returned flight ticket home - which I had - and I was let in. In my opinion, as long as you can prove to the officer that after the program is over it is her intention to return to her home country, I do not see why they would not let her in using her student visa which is a non immigrant visa in any case. Hope this is helpful. Good luck!