It’s always a smart idea to offer to write a letter of recommendation for a colleague who you know is well qualified for the job in question. Consider it good career karma; after all, you never know when you’ll need a letter of recommendation for yourself, and your colleague may be happy to return the favor.
Describe how you know the person. Give some context for your recommendation. Let the reader know how you met them, how you worked together, and your basic qualifications.