英文笔友该怎么回信?她是外国的,她隔了好久才给我回信,一个劲的在信中说对不起.我咋回阿!

当然是表示理解了原谅了.可以用类似与下面的句型：
I can totally understand you.It's much better to receive your letter later than never.Just be easy please.
我完全可以理解你,迟收你的信总比没有收到要好的多.请不必这么自责.