Is this argument wrong "since DOM is special kind of RDOM, then RDOM is NP-hard"? The domination problem $DOM$ is defined as
$$
DOM = \{ \langle G,k \rangle\ | \ G \text{ has a domination of size } k, K \in \mathbb{N} \},
$$
and the rainbow domination problem $RDOM$ is defined as
$$
RDOM = \{ \langle G,r,k \rangle \ | \ G \text{ has a r-dominating set of size } k; r,k \in \mathbb{N} \}.
$$
A $r$-rainbow domination for a graph $G=(V,E)$ is a mapping of the form  $f:V \rightarrow 2^{\{ 1,\ldots, r\}}$ such that $f(v) = \emptyset$ implies $\cup_{u \in N(v)} f(u) = \{ 1,\ldots , r\}$ where $N(v) = \{ u \ | \ \{u,v\} \in E \}$.  The size of $r$-domination is defined as $\sum_{v \in V} |f(v)|$.

To show that $RDOM$ is $\mathbf{NP}$-hard, one must show that $DOM \leq_P RDOM$, i.e. given an instance $\langle G,k \rangle$, construct an instance $\langle G',r,k' \rangle$ in polynomial time such that $\langle G,k \rangle \in DOM$ if and only if $\langle G',r,K' \rangle \in RDOM$.

My friend states that we know that a domination is a $1$-rainbow domination in the same graph, i.e. domination is a special kind of rainbow-domination, and since domination is $\mathbf{NP}$-hard, then $RDOM$ is also $\mathbf{NP}$-hard. How to convince him that this is wrong? or this argument is correct?
There are two statements here that rule the argument. The first statement:


  Statement 1.
  To show that $RDOM$ is $\mathbf{NP}$-hard, one must show that $DOM
 \leq_P RDOM$, i.e. given an instance $\langle G,k \rangle$, construct an instance
  $\langle G',r,k'\rangle$ in polynomial time such that $\langle G,k \rangle \in DOM$ if and only
  if $\langle G',r,K'\rangle \in RDOM$.


is a general framework for showing that RDOM is NP-hard. However, in this statement, you have not shown how to prove that this reduction works. You just said how to prove that this problem (or any problem) is NP-hard. In order to prove it, you need a proper reduction from all instances of DOM to instances of RDOM. The second argument however,


  Statement 2.
  By the way, my friend states that we know that a domination is a
  $1$-rainbow domination in the same graph, i.e. domination is a special
  kind of rainbow-domination, and since domination is
  $\mathbf{NP}$-hard, then $RDOM$ is also $\mathbf{NP}$-hard. How to
  convince him that this is wrong? or Is this argument correct?


looks like a proper reduction. If "every domination is a $1$-rainbow domination in the same graph", then this proof is completely right. This is a proper reduction from every instance of DOM problem to instances of RDOM or 1DOM problem. However, if "every domination is NOT a $1$-rainbow domination in the same graph", then this proof is completely wrong.

My understanding is that a domination, can be any of 1-rainbow domination, ...,or |V|-rainbow domination. In this case, it is implied that by having a polynomial algorithm for RDOM problem, you can iterate $|V|$ times thorough $r$-dominations and determine if the graph is dominating. Thus, if you have a polynomial algorithm for RDOM problem, you can solve the DOM problem polynomially.