The way that English is spoken you can use either form.
 
If you were going to extend the comparatives to then you would be best to use:
 
lovely - lovelier - loveliest
friendly - friendlier - friendliest 
 
It can be a little more awkward (although grammatically correct) to use the  �most � comparatives naturally for many non-native speakers as  �mos � is usually emphasised when used:
 
lovely - more lovely - most lovely
friendly - more friendly - most friendly
 
Generally a one or two syllable word will take  �-er �, many two syllable words can use either form & three or more syllebles will take  �more - �. 
 
Regards,
AB