Sorry for your loss 
Some ideas from [name_f]Frances[/name_f] and [name_m]Frank[/name_m]:
[name_f]Effie[/name_f] (as in the letter F, since it’s their initial)
[name_f]Frankie[/name_f] (I think it sounds nice with R and J)
[name_f]Fae[/name_f] / [name_f]Faye[/name_f] (similar letters and same initial)
[name_f]Annika[/name_f] [name_f]Faye[/name_f] (from some of the letters)
[name_f]Francine[/name_f]
[name_f]Fenna[/name_f]
[name_f]Fearne[/name_f] / [name_f]Fern[/name_f] (similar letters)