This line is wrong. The first parameter needs to be a string that looks like this:
SqlDataAdapter SDA = new SqlDataAdapter("SELECT * FROM Friends where FirstName = " + char.parse(textBox1.Text), con);
Obviously, Bob is your input. Build a string that looks like this and pass it to the adapter:
SELECT * FROM FRIENDS WHERE FIRSTNAME = 'Bob'
Now you obviously need to get textBox1.Text in there or you will only select Bob forever. Use th line from my first post to format you sql string according to your input.
string sql = "SELECT * FROM FRIENDS WHERE FIRSTNAME = 'Bob' ";
SqlDataAdapter SDA = new SqlDataAdapter(sql, con);
Databases are advanced stuff. Have a look at string formatting and SQL first.