Yes, my friend could come with us for the picnic.
There are two options to choose from in this situation - "can" and "could". In this context, both "can" and "could" are used to ask for permission or to make a polite request. However, "could" is considered more polite and formal than "can". Therefore, using "could" in this sentence would be more appropriate as it conveys a sense of politeness and respect towards the friend.

Explanation:
- Using "could" instead of "can" in the sentence shows politeness and respect towards the friend.
- By choosing "could", the speaker is making a polite request rather than a direct question.
- It is always important to be polite and considerate when inviting someone to join an activity or event.
- The use of "could" in this sentence reflects the speaker's courteous attitude towards their friend and their invitation to the picnic.
In conclusion, when inviting someone to join an activity or event, it is always a good idea to use polite language. By using "could" instead of "can" in the sentence, the speaker demonstrates their respect and consideration towards their friend.