Wa alaykum salam warahmatullah,
Fasting is completed at the time of maghrib prayers which occurs when the sun has set. You need to ascertain what was the sunset time and if the time when the sunni brother opened fast ( 9:03pm) was at sunset time or past sunset time?
If it was so then the fast is considered valid.
Note that, as a practice of precaution, it is usually a common practice in our shia communities to add some minutes (10 minutes) to the time of sunset. This may be a reason for the 'shia timings' at that location to be 9:18 as you have stated.
Our respected maraji, including Ayatullah Al Udhama Seyyid Ali Seestani, have always advised us about interacting with sunni brethren on basis of love and respect. An expected outcome of these interactions is the exchange of knowledge especially with regards to the variations in the beliefs and practices between each other. This will overcome the ignorance and prejudicemamongst each other. It is through strengthening this relationship that the rationale for the various differences in fiqh can be discussed and mutually appreciated without offending each other.
This scenario could have been an opportunity to enlighten the group on the shia fiqh with regards to the precaution time for maghrib and delay in opening fast. Consequently rathermthan compromising on our faith and practices so as to 'show solidarity' one needs to confidently practice our faith especially where there is no justification to abandoning it or modifying them. Solidarity is not about
unification, abandoning and compromising. Rather it is about accepting differences, respecting each other and appreciating diversity.
With salams and duas,