Some things for you to consider which might explain the lamps failing:
a. Are they running too hot because they are enclosed or there is inadequate ventilation around them? Too hot and their life will be shortened.
b. Are they correct lamp for the fitting - shape, size, glass type? What is the maximum wattage lamp for that fitting.
c. Are the lamps subject to vibration or knocks?
d. Are the glass globes down or up? Globe up and air convection will mean the lamp runs a little cooler than globe down.
e. When do they fail - when on or when first switched on? How often are the lamps turned on and off - the turning on event is when most stress is caused to the filament.
f. What is the state/quality of the light switches? Are they opening and closing quickly with good separation of contacts. Otherwise is there arcing so a clean make/break is not happening prolonging the warm up of the filament or extending its cool down.
g. Do the lamps in her home flicker or occasionally rise markedly in brightness? Perhaps DNO has N conductor problem? - but other homes would notice this.
h. What is the mains voltage? Are they being overrun which will shorten life.
i. Are there any old type choke ballast fluorescent fittings? What motor or transformer equipment is in use? Switching transients especially when such equipments are turned off.
j. What kind of supply - overhead or underground? If voltage transients on an overhead 4 wire pole supply are a problem the DNO often uses a length of cable between the pole and the ADS cutout instead of two strung conductors to attenuate these voltage transient wave fronts by a change in characteristic impedance. This length of cable has a lower characteristic impedance (aka surge impedance) than the overhead line's and so the voltage transient heading toward the home is diminished by the effect of the discontinuity/step down in impedance.
