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