I agree. Long-term, I would get more Lemon Tetra 9and Diamond Tetra too) if you have tank space. All characins should be in groups of no less than 6 and more is always better if space permits.
There is not the slightest doubt that a lone tetra will be highly stressed. Sometimes this is unavoidable (as fish naturally die off in a group, someone will be the last survivor, or the store may only have a couple), but with young fish one should not have them isolated if it can be helped. A couple weeks is OK, so long as you intend to add (or return it).