Biometrics For fiancé Visa - Mexico, HELP

Hi all, fiancée about to do fiancé visa application to come and settle with me in the UK, understand She will need to do some biometrics, issue is she is in Guadalajara and although they say there is a centre here I can’t see anyone who went there and experienced it, mostly advice to go to Mexico city instead. The issue is she will have to fly there and if they hold her passport how will she get back? We were thinking of the standard service also so wouldn’t want to be nervously staying in Mexico city for weeks and weeks. If you have any information on the centre on Guadalajara or if there is an option for her to retain her passport after biometrics whilst the application is being processed please please let me know !