We don't use "fortuitous[ly]" very often these days. I would use "Fortunately" or "Luckily". Of course, it is impossible to tell if that is the correct word without the context. Why was it fortunate/lucky that you just saw your best friend in the shopping center?
I was on a tram recently when inspectors asked to see my tram pass. I did not have it on me, but, just as they were about to fine me, my wife fortuitously boarded the tram; she had my tram pass in her handbag.